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Holographic Storage Takes a Step Toward Viability

Photonics Spectra
Oct 1998
Aaron J. Hand

If holographic storage is ever going to make it out of the lab and really compete with existing data storage techniques, researchers must come up with practical, cost-effective solutions to some of the hurdles holography still faces. One such hurdle is volatility.
Using polymers, researchers are able to fix information onto write-once, read-only holographic memories, but once they've made that data nondegradable, erasing or rewriting is impossible. On the flip side, lithium niobate has been shown to be a good material for rewritable holographic storage, but simply reading information from the material tends to erase the stored data.
Now a group of researchers at the California Institute of Technology has developed a process to resolve volatility issues in lithium niobate, edging rewritable holographic storage closer to practical reality. It uses iron and manganese electron traps to overcome the tendency of excited electrons within lithium niobate to migrate in a particular direction. Reading stored information requires using the same light wavelength that was used to record it, but this process causes uniform electron excitation, which, in turn, erases the original hologram.

Variable absorption
By doping a lithium niobate crystal with the two traps, the researchers can change the crystal's absorption depending on which wavelength they choose to illuminate it. A shorter wavelength makes the crystal absorptive for a wide range of visible wavelengths, and a visible light returns the crystal to its transparent state. What this means for holographic storage is that, if a crystal is illuminated with incoherent UV light during recording, the stored data can be read without erasure with a visible light.
Previous approaches to making storage materials nonvolatile have proved effective but not practical, requiring heating the storage material or applying large external fields. CalTech's method, however, does not have these requirements.
In a typical rewritable holographic storage system, a red laser would record information and a blue light-emitting diode could erase it, noted Demetri Psaltis, professor of electrical engineering and leader of the CalTech researchers. The storage medium could then be rerecorded with red and blue light simultaneously, he said.
Such a system is not likely to appear for several years, however, because the first systems on the market most likely will be simpler read-only, write-once memories. "Read-only holographic storage will have to start appearing in, at most, three years from now if it's going to sustain its momentum," Psaltis said. After this happens, the market should begin to open up to rewritable holographic systems, he added.

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